The present invention relates to hall sensors and, more particularly, to hall sensors implemented in a CMOS technology with an improved contact electrode geometry for the reduction of the offset signal.
In general, a hall sensor is constructed from an n-doped active semiconductor area on a p-doped semiconductor substrate. Usually, the n-doped active area is connected to an external control logic via four contact electrodes disposed diagonally opposite in the active area. The four contact electrodes are divided into two opposite control current contact electrodes provided to generate a current flow through the active area and, further, into two opposite voltage tapping contact electrodes provided to tap-off a hall voltage occurring perpendicular to the current flow in the active area as a sensor signal in the presence of an applied magnetic field.
Usually, in hall sensors with the arrangement described above, there are two known geometries for the active sensor area illustrated in FIGS. 5 and 6. FIG. 5 shows a square hall sensor 20 whose contact electrodes 22a-d are disposed in the corners of the active area 24, respectively. The shape of the contact electrodes 22a-d of those of this known hall sensor arrangement is normally square. During operation of the hall sensor 20, current is fed between two diagonally opposite contact electrodes 22a, 22c in order to be able to tap-off a hall voltage across the other two contact electrodes 22b, 22d in the presence of an applied magnetic field.
FIG. 6 shows a cross-shaped hall sensor arrangement 30 in the shape of a xe2x80x9cGreek crossxe2x80x9d in which the rectangular contact electrodes 32a-d reside at the end of a cross arm, respectively, wherein the rear boundary of the contact electrodes 32a-d is identical with the boundary of the active area 34. The width of the contact electrodes 32a-d corresponds to the width of the cross arm, i.e., the contact electrodes 32a-d extend over the whole width of the active area 34 in the respective cross arm. Analogous to the square hall sensor, during operation of the cross-shaped hall sensor, a current is fed between two opposite contact electrodes 32a, 32c in order to be able to tap-off a hall voltage across the two contact electrodes 32b, 32d in the presence of an applied magnetic field.
However, in CMOS processes for the production of semiconductor structures, inhomogeneties or defects in the semiconductor material of the active area occur often due to productional processes. These inhomogeneties cannot be fully avoided even with expensive production methods. These inhomogeneties are, however, often a reason for the ocurrence of an offset of the sensor signal. This means that at the contact electrodes where a hall voltage is tapped-off, a sensor signal is detected, even when no magnetic field is applied to the active area. This interfering sensor signal is referred to as the offset of the useful sensor signal or simply as the offset signal. If those inhomogeneties are in unfavourable positions in the active area, there can be a relatively high offset signal in the known hall sensor elements, since the current lines in the active area can change unfavourably, resulting in a local high resistance in the active area. Thereby the offset of the sensor signal occurring at the hall sensor element depends strongly on the number of inhomogenities and the position of said inhomogenities.
Due to this strong dependency of the offset signal from the inhomogeneties in the conventional hall sensor elements, large exemplary variations occur. Further, the sensitivity and the measurement accurracy of the hall sensors is strongly affected. For this reason, an offset compensation and a correct evaluation of the sensor signals in general, require very expensive circuit technology.
Starting from on this prior art, it is the object of the present invention to provide an improved hall sensor element with a reduced offset in the sensor signal.
This object is achieved by a hall sensor element according to claim 1.
The present invention provides a hall sensor element with two opposite power supply contact electrodes with an active area defined between them for generating a current flow through the active area, and with two opposite voltage-tapping contact electrodes for tapping-off a hall voltage, characterised in that a portion of the respective contact electrode facing the active area is formed in such a way that the interfering influence of the contact electrodes on the offset reduced effect of the spinning current operation is reduced.
The present invention is based on the realization that the offset of the sensor signals occurring in the hall sensor element can be strongly reduced by the appropriate choice, of geometry of the contact electrodes used. Fundamental for a small offset of the sensor signal is namely not only a homogeneous current density distribution of the control current in ideal conditions in the semiconductor material, i.e., without inhomogeneties from which the conventional sensor structures mainly result, but it is of much higher importance how the current density distribution changes because of present inhomogeneties or interferences in the semiconductor material of the active areas and, especially, at the contacts. At the same time, the resulting offset of the sensor signals should be as independent as possible from the position of the inhomogeneties or interferences in the semiconductor material in order to keep the variations of the resulting offset values low.
The advantages of the inventive geometries are based on the following context. The offset that is measurable from the outside of a hall element depends on three fundamental factors:
the strength and extension of the effect in the element;
the operational voltage at the element, and
the geometry of the element.
The first two factors are not to be considered any further in this context. In this case, there is only the geometry left for changes. The geometry has far-reaching and numerous influences on the properties of the element. A special connection exists between the contact geometry and the offset of the element reduced by the spinning current explained below.
The spinning current method consists of the fact that the measurment direction is constantly cyclically turned with a certain clock frequency by, for example, 90%, i.e., the operational current flows from one to the opposite contact electrode, wherein the hall voltage is tapped-off at the diagonally-opposite contact electrodes, whereupon in the next cycle, the measurment direction is turned by 90%. The measured hall voltages are summed-up wherein the offset voltages should almost cancel each other in one cycle, so that the portions of the signal that are really magnetic field dependent are left.
Even in the operation without spinning current, positions arise dependent on the chosen element and contact geometry where effects do not interfere and positions where they cause a large offset. A good example is the connecting line between the two control current contacts. Due to symmetry reasons, defects on this connecting line do not lead to an offset. As soon as there is a slight deviation from this line, however, there is immediately a measurable offset at the hall contacts, although the current densities at both points are almost identical and not negligibly small.
During the operation with spinning current such a sensitivity function arises across the location that describes the effects of a defect at a certain position on the offset, depending on the geometry. In the case of finite extended control or hall contacts this function looks relatively complicated. In a normal hall cross, the function possesses zero points on the connecting lines between respectively opposite contacts and, additionally, between the bisectors of the inner area of the cross. The remaining contour and thereby especially the extreme values of this function in the active area can be influenced by the appropriate choice of element and contact geometry. Usually, the locations with high relevance for the offset reside on the edges of the element and the contacts, while the inner area usually have no new local extreme values. Since all other properties of the element can also change due to modification of the geometry, all other properties need to be considered for changes at the contacts.
In the inventive contacts, the geometry was formed in such a way that a low offset with high yields and almost unchanged sensitivity could be achieved.
The effective resolution thereby increases according to the low offset of the element.